Polyvinyl butyral is generally obtained by a condensation reaction between butyraldehyde and polyvinyl alcohol. By varying the conditions of the reaction, the proportions of the starting materials, the molecular weight of the polyvinyl alcohol, and by incorporating various modification agents and plasticizers, it is possible to vary the properties of the final product to a considerable extent. In the manufacture of laminated glass having at least one sheet of polyvinyl butyral, the desired properties are particularly:
the compression creep of the polyvinyl butyral sheet; PA1 the impact resistance of the laminated glass having such a polyvinyl butyral sheet as interlayer; PA1 the adhesive force of the polyvinyl butyral to the glass sheets of the laminated glass (Pummel test); PA1 the transparency of the laminated glass, i.e., the absence of cloudiness or turbidity; PA1 the moisture resistance of the laminated glass.
Some of the known processes for synthesis of polyvinyl butyral make it possible to satisfy one or the other of the above requirements, but not all of them together.
In the published French Pat. No. 2,401,941, a process for preparing polyvinyl butyral was proposed that makes it possible to act at will on one, several or all of the properties listed above, and the resin so prepared is adaptable to the use of plasticizers which are less costly than those which are used when the polyvinyl butyral is prepared in the usual manner.
According to this process, all of the aqueous solution of polyvinyl alcohol, from 8 to 15% by weight, is placed in a reactor, an acid catalyst and an emulsifier are incorporated and butyraldehyde is introduced gradually over a period of 30 minutes while the initial mixture is stirred and the temperature maintained between 5.degree. and 12.degree. C.
After adding butyraldehyde, the resulting mixture is stirred for a period in excess of 30 minutes at a temperature of 8.degree. to 15.degree. C., then the temperature of the mixture is raised to a value between 60.degree. and 80.degree. C. over a period of an hour and a half to four hours. A base is then added into the mixture to raise the pH between 9 and 11 and the temperature is maintained between 60.degree. and 80.degree. C. for a period in excess of a quarter of an hour. The precipitated polyvinyl butyral is separated from the mixture and a very fine powder having a predetermined hydroxyl ratio is obtained.
The polyvinyl butyral sheet, plasticized with an appropriate plasticizer, exhibits the necessary properties, and particularly a satisfactory creep, so that it can be used in laminated glass. Nevertheless, it is desired to improve this creep.
According to French Pat. No. 1,314,959, polyvinyl butyral is produced by simultaneously introducing two streams into a reactor containing water at 90.degree. C. over a period of 75 minutes. Stream 1 is an aqueous solution of polyvinyl alcohol, an acid catalyst, a sequestering agent, and stream 2 is butyraldehyde. A surface-active agent is introduced into the reaction mixture after having added at least 60% of the polyvinyl alcohol and at least 50% of the buyraldehyde and preferably after having finished adding these ingredients and after having brought the temperature of the reaction to between 50.degree. C. and 100.degree. C.
The product obtained after neutralization and separation consists of large grains. These grains are poorly suited to plasticizing and the plasticized sheet is generally cloudy.